I have a joint between two sheets of cement board that forms a small ridge elevated maybe 1/8" relative to the centers of either board. I was wondering if extra thinset on both sides can be used to even it out or is it better to use self-leveling compound beforehand. I am using expensive marble tiles and don't want them to break to an unevenness.
You may break a few pieces(of marble) just picking them up.
A skim coat on either side of the joint will help.
Block feather with a concrete 'rubbing' block.
My marble floor always have Schluter Ditra under them. I also use it with any questionable floor.
Ceramic tile can tolerate L/360 deflection, stone is twice as sensitive at L/720.
If you have any doubts about floor springiness, use an isolation membrane or stiffen joists or add a beam. Cement board does little for stiffness.